1. Field of the Invention
This invention relates to a method for the production of a particulate hydrated gel polymer and an absorbent resin. More particularly, it relates to a method for the production of a particulate hydrated gel polymer, characterized by extruding a hydrated gel polymer possessed of a cross-linked structure through a perforated plate containing holes of a specific diameter at a specific temperature. It further relates to a method for the production of an absorbent resin by drying the particulate hydrated gel polymer described above and optionally disintegrating and/or pulverizing the dried polymer.
2. Description of the Prior Art
The absorbent resins heretofore known to the art include cross-linked polyacrylates, saponified acrylic ester-vinyl acetate copolymers, modified cross-linked polyvinyl alcohols, cross-linked isobutylene-maleic anhydride copolymers, and starch-acrylic acid graft polymer, for example. They have found extensive utility for sanitary absorbents such as sanitary napkins and disposable diapers and for water-retaining agents and dewatering agents in the agricultural and horticultural field and the civil engineering field.
As methods for the production of these absorbent resins, the methods disclosed in JP-A-56-161,408, JP-A-57-94,011, JP-A-57-158,209, and JP-A-198,714 have been known as resorting to the technique of reversed-phase suspension polymerization and those disclosed in JP-A-57-34,101, JP-A-48,42,466, JP-A-58-49,714, JP-B-59-37,003, U.S. Pat. Nos. 4,286,082, and 4,625,001 have been known as resorting to the technique of aqueous solution polymerization.
The methods by the reversed-phase suspension polymerization, because of the inevitable use of an organic solvent, have the possibility of not merely jeopardizing the shop environment but also threatening a fire or an explosion and dictate adoption of a countermeasure to preclude such adverse phenomena and, as a natural consequence, boost the cost of operation inclusive of the expense for the organic solvent and the expense for the removal of used organic solvent. Moreover, since the organic solvent persists in the finished product, though in a very minute amount, an effort to obtain perfect removal of the remaining organic solvent further adds to the cost. The absorbent resin which is produced by the methods resorting to the reversed-phase suspension polymerization comprises spherical particles of a small diameter. When this resin is used such as in a disposable diaper, for example, it has a problem of inconvenience of handling because the spherical particles are not retained fast in a fibrous absorption core such as of pulp but suffered to spill out.
In contrast, the methods by the aqueous solution polymerization are free from the problems mentioned above. The methods disclosed in JP-A-57-34,101 and U.S. Pat. No. 4,625,001 are directed to producing a cross-linked polymer by placing in a vessel provided with stirring blades the aqueous solution of a monomer destined to form a cross-linked structure and mature into a hydrated gel polymer in the process of aqueous solution polymerization and a polymerization initiator, initiating polymerization of the monomer therein, and finely dividing a hydrated gel polymer being formed in consequence of the progress of the polymerization with the shear force of the stirring blades generated by the rotation of the stirring blades and meanwhile continuing radical aqueous solution polymerization. These methods of production have the advantage of not merely operating ideally but also producing a finely divided hydrated gel polymer possessed of a cross-linked structure in the molecular unit thereof in a high yield. Even these methods, however, have sometimes disadvantage of producing only in a low yield an absorbent resin which has a high absorption ratio and a small water-soluble component.
It has been well known to persons of ordinary skill in the art that the absorption ratio is increased by lowering the cross-link density. It has been also known that the water-soluble component in the absorbent resin is large when the absorbent resin is produced by an operation involving a work of lowering the cross-link density. The water-soluble component, when the absorbent resin forms a hydrogel texture on contact with a liquid such as water, urine, or body fluid which is given to be absorbed, exudes from the affected area. The water-soluble component which is thus extracted by the liquid subjected to absorption not only lowers the absorption ratio but also aggravates the deterioration of the absorbent resin. This exudation entails the disadvantage of imparting a ropy unpleasant texture to the absorbent resin and contaminating the liquid under treatment.
A desire has been expressed, therefore, for a method which is capable of producing an absorbent resin enjoying a high absorption ratio and having only a small water-soluble component.
U.S. Pat. No. 4,654,039 and JP-A-1-144,404 disclose methods for producing an absorbent resin having a high absorption ratio and only a small water-soluble component by subjecting a monomer of a free acid type or a monomer having a specific neutralization ratio to aqueous solution polymerization. These methods are disadvantageous in necessitating an extra aftertreatment for neutralization, operating with complexity and poor efficiency, and imposing a restriction on the conditions of polymerization.
The hydrated gel polymer which is obtained by polymerization is generally pulverized after the step of drying and is consequently marketed in the form of a powdery product. Heretofore, for the purpose of ensuring efficient drying of this hydrated gel polymer, efforts have been directed to enlarging the surface area of the hydrated gel polymer to the fullest possible extent. Methods for crushing the hydrated gel polymer by extrusion through a perforated plate (JP-B-54-32,176, JP-A-50-136,348, etc.), for example, have been known. These conventional methods have been unable to produce a particulate hydrated gel polymer because the minute hydrated gel polymer particles resulting from the extrusion are suffered to agglomerate again and form cords.
Methods which rely on the incorporation of such additives as a lubricant for preventing the agglomeration of the pulverized hydrated gel polymer during the extrusion of the hydrated gel polymer through a perforated plate (JP-A-59-30,826, JP-A-59-119,172, etc.) have been known to the art. These methods, however, have at times occasioned the additive persisting in the polymer to exert an adverse effect on the quality of the product. A method which effects the pulverization of a water-soluble hydrated gel polymer possessed of no cross-linked structure by extruding this polymer through a perforated plate at a specific temperature (JP-A-54-106,568) has been known to the art. This method has entailed a problem of poor productivity because it requires to decrease the diameter of holes in the perforated plate for the purpose of enhancing the efficiency of drying of the hydrated gel polymer. In this method, the pulverization has produced no discernible improvement in physical properties.
No method has yet been established for producing a particulate hydrated gel polymer and an absorbent resin having a high absorption ratio and a small water-soluble component in high yields by a simple process without requiring any special device. A method for producing in a high yield a particulate hydrated gel polymer containing no additives such as lubricant, succumbing to no excess pulverization, and allowing a highly efficient drying remains yet to be established.
An object of this invention, therefore, is to provide a method for the production of a particulate hydrated gel polymer having a high absorption ratio and a small water-soluble component and an absorbent resin.
Another object of this invention is to provide a method for producing in high yields a particulate hydrated gel polymer and an absorbent resin both having a high absorption ratio and a small water-soluble component by a simple process without requiring any special device.
Still another object of this invention is to provide a method for producing in a high yield a particulate hydrated gel polymer containing no additives such as lubricant and succumbing to no excessive pulverization.